1. Field of the Invention
The present invention relates to a four-wheel steering apparatus for vehicles and, more particularly, to a four-wheel steering apparatus for vehicles adapted to change a turning ratio of the rear wheels to the front wheels.
2. Description of the Prior Art
A four-wheel steering apparatus for a vehicle designed to turn the rear wheels in association with the front wheels in response to the operation of a steering wheel by an operator includes a front wheel turning mechanism and a rear wheel turning mechanism, and a steering angle of the steering wheel is arranged so as to be transmitted through the both turning mechanisms to the front wheels and the rear wheels as a turning angle. The steering angle of the steering wheel may be transmitted to the rear wheel turning mechanism by way of various means such as mechanical, electrical, fluid ones or a combination therewith.
In the four-wheel steering apparatus, a turning ratio of the rear wheels to the front wheels (that is, a ratio of a turning angle of the rear wheels to that of the front wheels) is not always constant and is commonly variable on the basis of a predetermined turning ratio characteristics. the turning ratio characteristics is set using a running state of the vehicle as a parameter. The parameters may include, for instance, a vehicle speed, the steering angle of a steering wheel, and a transverse G (an acceleration in the transverse direction) acting on the vehicle. U.S. Pat. No. 4,313,514 discloses the turning ratio characteristics set on the basis of vehicle speeds.
The turning ratio characteristics is not necessarily limited to one kind. A plurality of turning ratio characeristics may be set and one of them may be selected by a manual selection or the like. U.K. Patent Publication No. 2,152,452 discloses delay means for changing turning ratios gradually in selecting the turning ratio characteristics.
The direction in which the rear wheels are turned may be determined by a so-called same phase direction and a so-called reverse phase direction. The turning in the same phase direction is defined here, on the one hand, as the turning of the rear wheels in the direction identical to the direction in which the front wheels are turned. The turning of the rear wheels in the reverse phase direction is defined here, on the other hand, as the turning of the rear wheels in the direction opposite to the direction in which the front wheels are turned. Accordingly, in instances where a turning ratio is altered from one state to another, a changing direction being in the same phase direction, on the one hand, means that the direction in which the rear wheels are turned approaches to the direction in which the front wheels are turned. For example, where the front wheels are turned left, the rear wheels are also turned in the left-hand direction from the existing position. A changing direction being in the reverse phase direction, on the other, means that the direction in which the rear wheels are turned is parting from the direction in which the front wheels are turned. For instance, where the front wheels are turned left, the reara wheels are turned in the right-hand direction from the existing position. And a direction for changing the turning ratio has nothing to do with an existing state in which the rear wheels are turned, that is, the state in which the turning state is currently in the same phase or in the reverse phase. More specifically, assuming the existing state than the front wheels are turned left while the rear wheels are turned right, the state in which the rear wheels are turned left from the existing state is called an alteration of the turning ratio in the same phase direction and the state in which the rear wheels are turned farther right therefrom is called an alteration of the turning ratio in the reverse phase direction.
The alteration of the turning ratio in the same phase direction, on the one hand, causes the vehicle to stay in a stabler state while the capability of turning the vehicle head is decreased. The alteration of the turning ratio in the reverse phase direction, on the other, improves the capability of turning the vehicle head while impairing the stability of the vehicle. For these reasons, the turning ratio characteristics is set so as to harmonize the vehicle head turning capability and the vehicle stability in an optimum manner. For instance, where the turning ratio characteristics is set using vehicle speeds as a parameter, the turning ratio characterisics is set so as to allow the turning ratio to be changed farther in the same phase direction as vehicle speeds get higher from the viewpoint of ensuring the vehicle stability during the time of travelling at high speeds as well as the vehicle head turning capability during the time of travelling at low speeds, as disclosed in U.S. Pat. No. 4,313,514.
Extensive studies have been made to put the four-wheel steering apparatus into practice. As a result, it has been found that a rate of speeds at which the turning ratio is changed lies one of the points for commercialization.
Heretofore, a speed of changing the turning ratio has been set so as to be equal in both instances where the turning ratio is changed in the same phase direction and where the turning ratio is changed in the reverse phase direction. Therefore, if the speed of changing the turning ratio in the same phase direction is set to get higher from the viewpont of ensuring the vehicle stability faster, the vehicle stability will be impaired radically in changing the turning ratio in the reverse phase direction. On the contrary, if a speed of changing the turning ratio in the reverse phase direction is set so as to get delayed in order not to impair the vehicle stability radically, that is, in order for an operator (driver) to adequately cope with an alteration of the vehicle in an unstable direction, it may be delayed to ensure the vehicle stability in altering the turning ratio in the same phase direction.
In particular, if the turning ratio characteristics is set using vehicle speed as a parameter, as have been described above, the turning ratio is changed in accordance with acceleration or deceleration regardless of the fact that a steering wheel is not operated at all. Thus the turning ratio is changed without operation of the steering wheel because many things may occur beyond expectation of an operator. From such a point of view, too, what has been described above may cause problems. These may also be applicable to all the cases where the turning ratio is changed notwithstanding the transverse G or the like, or no operation of the steering wheel.